Just how much does your metabolism hinder or help your weight – and what can you do about it? We sort the facts from the fiction.
Many of us are quick to attribute our weight problems to a slow metabolism. But what exactly is your metabolism? And what can you to do to make it work for you?
“Metabolism is one of the most common reasons women give for their expanding waistlines, especially once they hit their 40s,” says nutritionist Juliette Kellow. "And, although it’s not the whole answer, it’s a big factor — our metabolism does start to drop as we get older, making it harder to control our weight.”
Metabolism: your body’s engine
Understanding how your metabolism works and what affects it is crucial if you want to give it a boost in order to help manage your weight. “Think of your metabolism as your body’s engine. It is constantly burning fuel — or kilojoules — to keep it ticking over. Most people are surprised to discover they actually burn most of their kilojoules 'at rest'”, says Juliette. Our metabolism is the total amount of kilojoules our body burns simply to keep us alive and get us through the day. It is made up of:
Resting metabolic rate (RMR), energy burned through things like breathing and keeping our heart beating, accounting for 60–75% of our metabolism
The thermic effect of food, which is the kilojoules we use up when we eat, digest and process food, accounting for around 5–15% of our metabolism
Both planned and incidental exercise.
There are several factors that influence how efficient your metabolism is.
“Some of these, such as your age, gender and genes, are beyond your control,” says Juliette. As you get older, your muscle mass decreases, and the less muscle you have, the slower your metabolism. That’s why men, who naturally have more muscle than women, tend to have a faster metabolism and are often able to eat a little more without gaining as much weight. While you can’t blame your genes entirely, some of us are born with a faster metabolism and others have to put in more effort to keep it revving. It's how you do this that can generate confusion.
Factors that can affect your metabolism
some medical conditions
Factors that can affect your weight
genetics (including body type, metabolism, and hormones)
True or false?
Eating lots of small meals is better for my metabolism
During the hours you are awake, it’s helpful to eat regularly, which gives your body a slow and steady supply of fuel to use and helps regulate your blood glucose levels. However, there is no conclusive evidence that eating more frequent small meals is better for your metabolism than three meals a day, according to a review in the British Journal of Nutrition. It seems any effect on body weight is the result of total energy intake, rather than different meal patterns.
There is no set number of meals and snacks you should eat in a day — it really depends on your preference. Remember starting the day with breakfast is always recommended. You may find, however, that including small snacks will prevent you from overeating at your next meal because you are starving, thus decreasing your total intake for the day and helping to control your weight.
Certain foods, drinks or supplements can boost my metabolism and help me burn fat
You may have read that things like coffee, green tea and chillies can speed up your metabolism, more than the usual thermic effect of food that occurs after a meal. While this is true to some extent, the increase from these foods is very small and unlikely to produce a noticeable difference in your weight. For example, a study published in Obesity Reviews found that drinking five cups of oolong tea, green tea or a caffeinated beverage increased total energy expenditure for the day by only 98.8kJ, 48.8kJ and 38.3kJ respectively. Another study published in Lipids found that adding 10g of chilli to a meal increases energy expenditure by 20–60kJ per meal, depending on the chilli’s heat. In other words, you can include them in a healthy diet, but don’t rely on these foods to help you lose weight.
There are also many supplements that claim to speed up your metabolism. These include carnitine, thyroid supplements, fat burners and caffeine, as well as natural substances like ginseng, green tea and nettle extracts – or combinations of all of these. But the truth is, if you are looking for long term results, there’s little these products can do for you.
A review of the evidence for a range of metabolism-boosting supplements in Obesity Reviews concluded that: “For most supplements, there is a lack of scientific data. Caffeine and green tea have evidence that they may enhance metabolism. However, effects in humans have generally been small... “Although some others show potential to enhance metabolism...conclusive evidence is lacking.”
As well as being expensive, in some cases supplements can even have harmful side-effects such as liver damage.
No food or supplement will significantly alter your metabolic rate. A proven and safe way to boost your metabolism is to increase your muscle mass. To do so, make sure you include resistance or strength training in your exercise routine several times a week. See an exercise physiologist if you’re not sure where to begin. And, if you are thinking about taking any supplements, see your doctor.
Stress boosts my metabolism
The stress response temporarily increases your body’s metabolism and suppresses appetite. Sudden stress initiates the ‘fight or flight’ response, in which your body releases adrenaline and cortisol. These hormones provide a rush of glucose and oxygen to your brain and muscles, and stimulate fat and carbohydrate metabolism for fast energy so that you are ready to fight or run from the danger. But when you are constantly under stress, the continuous release of cortisol can have detrimental effects. Cortisol can stimulate appetite and suppress your immune system. Research has suggested it can cause fat deposits in the abdominal area, increasing your risk of chronic disease.
Stress gives your metabolism a short term boost to cope with life-and-death situations, but prolonged or chronic stress can lead to increased appetite and weight gain. Managing stress is important for both weight management and long term health.
I’m overweight because I have a slow metabolism
It’s unlikely – there are several factors at play here. Larger people generally have a higher metabolism than smaller people because the bigger you are, the harder your body has to work with every movement you make. On the other hand, people who are slim and very muscular will also have a relatively faster metabolism than people with less muscle because increasing muscle mass increases metabolic rate.
A small percentage of people have a medical condition (such as hypothyroidism or Cushing’s Syndrome) causing a slower metabolism, so they carry more weight than you might expect, but most of us can’t blame our metabolism for excess weight.
Some people have naturally faster metabolisms
We are all different, and so are our metabolic rates; they vary depending on our genetics, gender, age and body composition. So it is true that two people may eat the same amount of food and it will affect them differently.
Those who have a higher muscle mass will also have a faster metabolism – either naturally or from exercise. Your body type can be in part determined by your genetics – whether you’re naturally muscular, slim or heavier. But it can also be determined by the type, amount and intensity of exercise you do. So in effect, it is possible to go against your genetic makeup to a certain extent.
Are there conditions that can slow down or speed up metabolism?
There are many medical conditions that can affect your metabolism, two of the more common being hyperthyroidism, where the thyroid gland is overactive and metabolism is faster, and hypothyroidism, where the thyroid is underactive and metabolism slower. Some medications may also affect appetite, or the way your body uses fuels or stores body fat. These include steroids, insulin and drugs for psychological health issues. Consult your GP if you think you could have a medical problem like hypo- or hyperthyroidism or have concerns about medications you are taking.
Five ways to boost your metabolism
1. Move it to lose it
The more you move the better! Studies show that fidgeters burn more kilojoules than people who sit still for long periods of time. A study in the American Journal of Clinical Nutrition showed that if you fidget while sitting you burn around 54kJ per minute compared to sitting still, which burns only around 5kJ per minute and standing, which burns around 13kJ per minute. So if you work in an office, crank up your total energy expenditure by moving regularly – walk to a colleague’s desk instead of sending an email, take the stairs instead of the lift or even take up an annoying foot-tapping habit while sitting at your desk! At home, stand up while you do the ironing or talk on the phone.
2. Do strength training
Use weights or do body weight exercises like lunges and squats, which will increase your muscle mass. The more muscle you have, the more kilojoules your body burns — even when it’s doing nothing (that's your RMR).
Fat cells, on the other hand, are a dead weight — they don’t burn many kilojoules. So be sure to include strength training as well as aerobic exercise in your workout regime. You could also try things like resistance bands, pilates or aqua aerobics.
Strength training is particularly important if you are trying to lose weight. The downside to shedding kilos is that most of us lose lean muscle as well as fat – up to a quarter of the weight lost, according to some research. To keep as much muscle as possible and therefore keep your metabolism up to speed, the solution is to reduce your kilojoule intake moderately, rather than excessively (for example, by only 2000kJ a day) and build strength training into your weight-loss plan at the same time.
3. Go for the burn
There’s an enormous amount of evidence that regular cardiovascular or aerobic exercise such as running, jogging or swimming (anything that gets the heart and lungs working harder) maintains lean muscle mass, hence a more efficient metabolism. Research also shows our metabolism stays raised even after we exercise and can remain elevated for up to 24 hours after a workout. For this to happen, we need to be exercising at least 40 minutes at a moderate to high intensity.
4. Don't skip breakfast
According to various studies, and confirmed by a review of studies with over 3500 children and 43,500 adults, people who eat breakfast tend to be slimmer than those who don’t. Eating in the morning can actually prevent weight gain, as researchers believe it gets our metabolism into gear for the rest of the day. When you wake up, your body may have gone 10 to 12 hours without food, depending on when you had your evening meal, so if you don’t eat breakfast, it thinks you are denying it kilojoules and goes into ‘starvation mode’. Not eating breakfast mimics a potential starvation situation, which is your body’s way of saving itself in the event of a real famine. When your body is starved, its metabolism slows down to conserve kilojoules, until food becomes regular again.
5. Ditch the crash diets
Don’t severely reduce your food intake in the hope you will quickly become slim. You won’t. In fact, drastic dieting will probably have the reverse effect. In response to an inadequate number of kilojoules, your body simply starts to use muscle rather than fat to provide it with energy. This has the effect of slowing down your metabolism and conserving energy so that you need fewer calories to survive – a result of early evolution so that the body could cope when food was scarce. Unfortunately, when you go back to eating normally, your metabolism will have become slower so your weight will increase quickly, and then you’ll struggle to lose it. It does recover, though, so put a stop to the cycle and make sure you only reduce your kJ intake moderately, eat regular balanced meals and exercise regularly.